Modern furnaces are designed to operate using much lower excess air percentages (5-10%) as compared to old models which are usually designed to operate at ~20% excess air or more. List down some...

Modern furnaces are designed to operate using much lower excess air percentages (5-10%) as compared to old models which are usually designed to operate at ~20% excess air or more. List down some consequences of a combustion operation having a) too little excess air, and b) too much excess air.